Visible light lasers, particularly blue and green lasers, are more desirable than infrared lasers for certain applications such as optical storage, printing and displays, because of their operational wavelengths. Using a laser operating at a shorter wavelength has advantages in a number of applications. For example, an optical disc can store approximately four times more information on the disc if a blue laser operating in the visible wavelength range is employed for the read/write functions rather than the infrared laser, as currently used, because the storage space on a disc is roughly proportional to the square of the wavelength. Also, laser printers using blue lasers could print with a higher resolution than printers using an infrared laser.
While infrared laser diodes have been known and utilized for a number of years, technological difficulties are still encountered for diode lasers emitting in the blue/green range. Researchers have focused on using high bandgap semiconductor materials, such as zinc selenide (ZnSe), a II-VI semiconductor material, and more recently on gallium nitride (GaN, a III-V semiconductor) for fabricating blue diode lasers.
An alternative approach to obtain blue or green light is to pass infrared light, generated by an infrared laser diode which is fabricated from III-V semiconductor materials, through a nonlinear crystal which converts the frequency of a portion of the infrared light to a larger frequency. The doubling of the frequency is discussed herein as an example of this frequency conversion. The infrared spectrum runs from approximately 780 nm to more than 10 microns while the spectrum for blue light is approximately 400 nm to 490 nm and the spectrum for green light is approximately 490 nm to 560 nm. Since doubling the frequency is equivalent to halving the wavelength of the light, when an infrared laser light passes through a frequency doubling optical crystal, a blue or green laser (which is half of the wavelength of the infrared light) is emitted from the waveguide output. The infrared light is referred to as the "fundamental wave" and the blue or green light generated by the frequency doubling is the "second harmonic wave". The process of frequency doubling is also commonly referred to as second harmonic generation ("SHG"). SHG can be realized in the employment of an optical waveguide to increase the efficiency of frequency conversion.
For an optical waveguide to emit blue or green light at the end of the waveguide, the fundamental and second harmonic waves propagate the entire length of the waveguide. The length over which these waves must propagate is the "propagation length".
Frequency doubling requires that the fundamental and second-harmonic waves stay in phase as much as possible in a nonlinear crystal or film, which creates the frequency doubling. Because the index of refraction in the nonlinear materials, including nonlinear optical waveguides, differs for the different wavelengths of the fundamental and second-harmonic waves, the light waves travel through the nonlinear optical waveguide at different speeds. The second harmonic wave, therefore, grows and, concomitantly, the frequency doubled light grows in power and intensity, until the point at which the two waves are out of phase by .pi.. The crystal length over which the waves fall out of phase by .pi. is the "coherence length". The second-harmonic wave would begin decaying towards zero amplitude after the first coherence length and would be of zero amplitude when the phase difference is 2.pi., that is, two coherence lengths. The same mechanism will be reproduced throughout the propagation length. If the coherence length is a few microns, the harmonic power generated is too small for practical use.
A particular optical nonlinear coefficient, [d.sub.ij ], is associated with each nonlinear optical material. A list of nonlinear coefficients for various crystals appears in the published work A. Yariv, "Quantum Electronics", at page 387 (J. Wiley & Sons, 1989).
To keep the fundamental and second harmonic waves in phase, the birefringence properties of the nonlinear material can be used which allows the maintenance of the fundamental and harmonic waves in phase and is used for frequency doubling using bulk materials. However, because of the necessary orientation of the crystal, birefringence uses a nonlinear coefficient of the crystal, e.g., d.sub.31, d.sub.32, rather than the larger d.sub.33 nonlinear coefficient of the crystal, which exists parallel to the +z orientation (for example, for LiNbO.sub.3 : d.sub.31 .about.5-6 pm/V, while d.sub.33 .about.34 pm/V). In addition, in thin film waveguides, it is not always possible to phase match the two fundamental modes. Modal dispersion can be used to realize phase matching between modes of different order. However, because the overlap between the two modes is very low in general, the phase matching achieved using modal dispersion is not efficient.
Therefore, in frequency doubling materials, such as lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3), the alternative technique of quasi-phase matching (QPM) is used to impose a modulation or periodic nonlinear coefficient patterning, one type of which comprises a polarization reversal pattern. This type of patterning is also referred to as a "poling pattern", which is formed in the doubling bulk crystal. Using this approach, the ferroelectric polarization of a ferroelectric crystal is periodically reversed, thereby creating a grating or pattern in the crystal comprising alternating positive and negative domains. The sign of the nonlinear coefficient of the crystal in one domain is reversed with reference to adjacent domain(s) having the opposite sign. For the polarization reversal to be most efficient, periodic poling is performed at a period along the crystal corresponding to twice the coherence length, l.sub.c, where l.sub.c =.pi./.DELTA..beta. with .DELTA..beta. equal to the difference in propagation vectors of the fundamental and second harmonic and with each domain being of equal width. This is known as first-order QPM.
Quasi-phase-matching is most efficient where there are abrupt reversals in domains which extend vertically through the portion of material to be used as the waveguide, creating vertical walls or boundaries between the domains. In cases where the domain boundaries are at an angle, i.e., nonperpendicular to the longitudinal extent of the waveguide, the frequency conversion efficiency is reduced. Only a portion of infrared light can be converted to blue or green light by the waveguide because the half period of the periodic nonlinear coefficient pattern will not be consistently equal to an integer multiple of the coherence length since the duty cycle of the periodic nonlinear coefficient pattern is a function of waveguide medium depth. The mount of blue or green light that is emitted divided by the amount of infrared light entering the waveguide equals the conversion efficiency of the waveguide in creating the desired blue or green light.
The second harmonic generation (SHG) power of the blue or green light generated in the nonlinear waveguide is proportional to the square of the propagation length and is also proportional to the square of the nonlinear coefficient of the material used in the waveguide. Therefore, to increase efficiency of such a crystal, one should create as long a propagation length as possible, and use a nonlinear material with a high nonlinear coefficient.
It is also desirable to confine the light to as small a region as possible because the power of the visible light generated is inversely proportional to the square of the intensity of the infrared light. The SHG efficiency is, in turn, proportional to the infrared power density. Therefore, using a waveguide made with thin film technology to confine the light, instead of directly coupling the light into a bulk crystal material, can significantly enhance the SHG efficiency.
There are a number of conventional, alternative methods for creating a periodic nonlinear coefficient pattern in a crystal to provide periodic poling such as in crystal bulk materials. Examples of such methods are titanium indiffusion in LiNbO.sub.3, proton exchange followed by an anneal in LiTaO.sub.3, and by heat treatment. See E. J. Lira et at., "Blue Light Generation by Frequency Doubling in Periodically Poled Lithium Niobate Channel Waveguide", Electronics Letters, Vol. 25(11), pp. 731-732 (1989); K. Mizuuchi et al., "Domain Inversion in LiTaO.sub.3 Using Proton Exchange Followed by Heat Treatment", 75 J. Appl. Phys., Vol. 75(3), pp. 1311 et seq., (1994); and K. Nakamura, et al., "Ferroelectric Domain Inversion Caused in LiNbO.sub.3 Plates by Heat Treatment", Applied Physics Letters, Vol. 50(20), p. 1413 et seq, (1987). It is also known to achieve periodic poling in thin oxide films by applying an electric field and then using these poled films as waveguides. See, for example, W. K. Burns et at., "Second Harmonic Generation in Field Poled, Quasi-Phase-Matched, Bulk LiNbO.sub.3 ", IEEE. Photonics Technology Letters, Vol. 6(2), pp. 252-254 (February, 1994); T. A. Rost et al., "Electrical Switching in Lithium Niobate Thin Films", International Symposium on Integrated Ferroelectrics (ISIF), 1991; F. W. Ainger, "Ferroelectric Thin Films by Metal Organic Chemical Vapour Deposition", Material Research Society Symposium Proceedings, Vol. 200, p. 37 et seq.
However, these poling techniques can be difficult to apply in periodic nonlinear coefficient patterning of thin films. For instance, the underlying substrate (perhaps, already containing a formed semiconductor laser) cannot be heated to an arbitrarily high temperature. More particularly, titanium indiffusion must be performed at about 1000.degree. C. in LiNbO.sub.3. However, the substrate on which the thin film has been deposited, or the film itself, may not sustain such high temperatures. Also, for some of the bulk crystal techniques, the material has to be multi-domain or uniformly poled in one particular direction before periodic nonlinear coefficient pattering, which may be difficult to control during the growth of the thin film. As for electric field induced poling, this process may be useful in bulk crystal material. However, if the film has been deposited on an insulating material, such as a sapphire substrate, field poling becomes more difficult to apply because as most of the voltage drop is across the insulating substrate or low dielectric sublayer so that there is no way to remove or dissipate the charge developed from the applied field in order to achieve the desired ferroelectric phase in the material.